Reimei observation of highly structured auroras caused by nonaccelerated electrons

Abstract

[1] We present observations of highly structured, thin auroras cased by precipitation of nonaccelerated electrons on the basis of optical and particle measurements performed by the Reimei satellite near the equatorward edge of the main auroral oval. The aurora has the following characteristics: (1) A full width at half maximum (FWHM) value is as low as only ∼1.8 km from optical measurements, and ∼0.6 km from particle measurements at the ionospheric altitude, which is much smaller than previously determined. (2) The FWHM value of 0.6 km corresponds to 9 km in the equatorial plane, which is ∼10 times smaller than the gyroradius of typical protons trapped in the near-Earth plasma sheet. (3) At high energies greater than ∼1 keV, the velocity distribution function of precipitating electrons is comparable to that of the trapped ones and does not demonstrate any plateau or positive gradient in the distribution. (4) The aurora was observed in geomagnetically quiet condition. (5) A geosynchronous satellite observed a significant increase in the plasma pressure of hot electrons in comparison with that of hot ions. The structuring of the aurora may be attributed to scattering processes of hot electrons. If this were the case, hot electrons would be scattered by whistler mode chorus, or electrostatic electron cyclotron harmonic waves, and the structured aurora would be a visual manifestation of the highly structured, cold plasma that determines the growth of the waves scattering the hot electrons.Citation: Ebihara, Y., T. Sakanoi, K. Asamura, M. Hirahara, and M. F. Thomsen (2010), Reimei observation of highly structured auroras caused by nonaccelerated electrons,

Spatial characteristics of wave-like structures in diffuse aurora obtained using optical observations

“…This suggests that electrons were not accelerated but instead scattered due to pitch angle diffusion. Thus enhanced precipitation of the more energetic electrons of 2-6 keV energy is responsible for the diffuse aurora, in accordance with some previous studies (Ebihara et al, 2010;.…”

“…Ebihara et al (2010) suggested that the diffuse aurora they observed was the visual manifestation of highly structured cold plasma in the magnetosphere. The existing structures could affect the wave-particle interaction causing precipitation of newly injected more energetic electrons.…”

“…At first it was believed that these structures were small-scale discrete auroral structures, caused by accelerated electrons, but later studies show that they in fact are variations of the modulation of the diffuse auroral precipitation. Ebihara et al (2010) and , among others, have reported observations of smallscale structures in diffuse aurora caused by precipitation of non-accelerated high-energy electrons.…”

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

“…For example, one of the causes for the morphology was suggested to be the modification of the electric field in the ionosphere [ Hosokawa et al , 2010]. In addition to the ionospheric feedback, highly structured diffuse auroras in the non‐patch shape were suggested to be a result of interchange instability in the magnetosphere [ Ebihara et al , 2010]. A Rayleigh‐Taylor type instability was also assumed to be the cause of finger‐like patches [ Shiokawa et al , 2010].…”

“…Precipitating and trapped electrons obtained from particle measurements showed two different peak energies and anisotropy between downward and trapped ones with higher energy, which suggested that the auroral stripes and background generated by the upper band chorus and ECH, respectively. Narrow field of view imagers on the REIMEI satellite captured “smokelike auroras” images with a width of 0.6 km (FWHM) estimated by using simultaneous particle measurements [ Ebihara et al , 2010]. This spatial feature probably resulted from the interchange instability maintained mainly by pressures of high‐energy electrons and pitch angle scattering by whistler mode waves generated in the dense plasma region.…”